C.1.20 Changes for hierarchical objects

Meshes and lights can now properly be used as hierarchical objects with the
same API. Here is how it works: meshes and lights implement the new
iSceneNode interface. You can do QuerySceneNode() to get the
scene object from the mesh, light, or camera without having to do
scfQueryInterface<>. To make some object a child of another you simply
do: